Kurt Gerstein was a member of the Nazi Party, but was expelled in 1936 when a search of his home revealed a stash of thousands of pamphlets deemed critical of Nazi policies. After briefly being imprisoned at the Welzheim concentration camp for activities against the state, he applied to join the German SS, and was accepted as a "repentant" German. Privately, though, Gerstein told his pastor that he had joined only to discern who was giving the orders within the German military hierarchy, in hopes he could eventually testify against them in court. Assigned to the SS Hygiene Department, he was involved in the design of decontamination procedures for the bodies and effects of persons killed in the gas chambers. In 1942 he was sent to witness a "demonstration" of the process at the Belzec death camp, where he saw several thousand Jewish men, women, and children executed.

Returning on the train, Gerstein cornered a fellow passenger, Swedish official Göran van Otter, described in vivid detail what he had witnessed, and begged van Otter to relay this information to Allied forces, in hopes that they in turn would drop leaflets to inform the German people of the atrocities. Van Otter relayed the message to the Swedish government, but the story was deemed "too bizarre" to be credible, and no action was taken. Gerstein also told Archbishop Cesare Orsenigo, the papal nuncio in Berlin, what he had seen, and Orsenigo ordered him out of his office. Gerstein wrote in his diary, "...the very representative on Earth of Jesus refused even to hear me, although tens of thousands of people were being murdered every day". He told others of the atrocities, including the press attaché at the Swiss Legation in Berlin and two more Catholic officials, all to no avail.

Gerstein remained with the SS, but used his position to sabotage several shipments of Zyklon B gas pellets and other chemical compounds needed at the death camps, while for his own safety he had to allow numerous other shipments to pass unfettered. In April 1945, Gerstein turned himself in to French authorities in the town of Reutlingen, Germany, and almost eagerly began telling them everything he knew about the operation of German death camps. He asked repeatedly whether anything had come from his contacts with the Swedish and Vatican representatives, but Allied officials saw Gerstein as complicit in the crimes he described. He was jailed at Cherche-Midi Military Prison in Paris, and when he was told he would be prosecuted as a war criminal, Gerstein hung himself in his cell.

In 1950, the German Denazification Court considered his case posthumously, and neither exonerated him nor found him to be a war criminal. "Taking into account the extenuating circumstances noted the court has not included the accused among the main criminals but has placed him among the 'tainted' ..." Gerstein's dilemmas were the basis of Saul FriedländerKurt Gerstein: The Ambiguity of Good, Rolf Hochhuth's 1963 play The Deputy, Thomas Keneally's Either Or, and the 2002 Costa-Gavras film Amen.